Phase Center Variations in Adaptive GNSS Antenna Arrays

Adaptive antennas with digital beamforming is a promising technology for mitigation of radio interference and multipath effects in GNSS receivers. It is however well known that adaptive antennas may introduce biases in the pseudo-range and carrier phase measurements of a GNSS receiver. This effect can be interpreted as the deviation of the array phase centre from its nominal position. Since beamforming is the process of complex weighting of array outputs, the introduced code and carrier biases are determined by the magnitude and phases of the array weights and therefore also depend on the weight control algorithm used. Also, the properties of a single antenna element and electromagnetic coupling between elements cause the variation of the array phase centre with the signal direction. Particularly in small arrays the radiation patterns of array elements can significantly differ from each other because they see different neighbourhoods. The mutual coupling between elements depends also on the incident angle of incoming signals. In microstrip antenna arrays, a part of the energy is radiated in the directions of the array edges and it may significantly disturb the radiation pattern of the array. All these effects have influence on the position of the array phase centre and must be accounted for by electromagnetic (EM) simulations. The variations of the array phase centre and the associated measurement biases should be known in GNSS applications with high accuracy. In this paper, we investigate the phase centre variations of a 2x2 uniform rectangular adaptive antenna array that is part of the demonstrator of a GPS/Galileo array receiver. The array is built up of antenna elements optimised for the reception of GPS/Galileo signals in the L1 band with a simultaneous suppression of interference from 1710–1785 MHz GSM band. For enabling realistic simulations, the radiation patterns of the antenna elements are obtained by full-wave electromagnetic simulations with Ansoft Designer that take into account the mutual coupling. We investigate the phase centre variations with the following beamforming techniques: the unconstrained adaptive null steering, the linearly constrained minimum variance (LCMV) and reference signal LMS beamforming. The simulations of the beamforming techniques are performed with the help of DLR in-house software multi-antenna GNSS receiver.